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| -rw-r--r-- | compiler/parser/Parser.y | 44 |
1 files changed, 44 insertions, 0 deletions
diff --git a/compiler/parser/Parser.y b/compiler/parser/Parser.y index 112c4a9b44..befd52fd7c 100644 --- a/compiler/parser/Parser.y +++ b/compiler/parser/Parser.y @@ -316,6 +316,48 @@ correctly, see the README in (REPO)/utils/check-api-annotations for details on how to set up a test using the check-api-annotations utility, and interpret the output it generates. +Note [Parsing lists] +--------------------- +You might be wondering why we spend so much effort encoding our lists this +way: + +importdecls + : importdecls ';' importdecl + | importdecls ';' + | importdecl + | {- empty -} + +This might seem like an awfully roundabout way to declare a list; plus, to add +insult to injury you have to reverse the results at the end. The answer is that +left recursion prevents us from running out of stack space when parsing long +sequences. See: https://www.haskell.org/happy/doc/html/sec-sequences.html for +more guidance. + +By adding/removing branches, you can affect what lists are accepted. Here +are the most common patterns, rewritten as regular expressions for clarity: + + -- Equivalent to: ';'* (x ';'+)* x? (can be empty, permits leading/trailing semis) + xs : xs ';' x + | xs ';' + | x + | {- empty -} + + -- Equivalent to x (';' x)* ';'* (non-empty, permits trailing semis) + xs : xs ';' x + | xs ';' + | x + + -- Equivalent to ';'* alts (';' alts)* ';'* (non-empty, permits leading/trailing semis) + alts : alts1 + | ';' alts + alts1 : alts1 ';' alt + | alts1 ';' + | alt + + -- Equivalent to x (',' x)+ (non-empty, no trailing semis) + xs : x + | x ',' xs + -- ----------------------------------------------------------------------------- -} @@ -665,6 +707,8 @@ body2 :: { ([AddAnn] :(fst $2), snd $2) } | missing_module_keyword top close { ([],snd $2) } + + top :: { ([AddAnn] ,([LImportDecl RdrName], [LHsDecl RdrName])) } : importdecls { (fst $1 |